By Jacques Coderre, Universal Instruments Corp., Binghamton, N.Y.
Worldwide production of magnetic storage devices, the prevailing computer media in use, has risen steadily at a yearly rate of around 20%, and that pace is projected to continue. Indeed, the annual production of magnetic disk drives increased from about 145 million units in 1998 to 170 million units in 2000, and is projected to reach 250 million in 2003.(1) This absolute growth in units does not, however, reflect the continuous rise in storage density that has increased by an order of magnitude over the last decade, due to significant breakthroughs in storage technology. Growth in Storage Capacity Annual growth of more than 100% in areal density (a measure of storage capacity per unit area) has been recorded, with areal densities on the order of 100 Gbit/ sq.in. projected by 2005.(1,2) In parallel, the price of storage has been steadily decreasing by 40% every year.(3) In addition to the technological complexity of read/write heads and media, the disk drive assembly includes leading edge packaging technology, such as high-density board assembly and advanced flex cable assembly. The challenges associated with such flex assemblies are addressed in this paper. A typical hard-disk-drive assembly consists of the drive, a circuit board controlling the heads, spindles and actuators and an interconnecting flex cable.
Pre-Amplifier Evolving Of all the electronic components used in a drive assembly, the pre-amplifier device has been evolving, along with the drive itself, as a critical player in the performance of the storage system. The pre-amplifier receives data from the read/write head and then amplifies the signal on its way to the controller circuitry. From a packaging perspective, new, high-performance heads require that the pre-amplifier device be placed closer and closer to the read/write head. In previous generations, the pre-amplifier was packaged as a traditional surface-mounted component and mounted on the main electronics board. Subsequent generations of high-performance disk drives required that the pre-amplifier be mounted directly to the flex cable, closer to the read/write head. Newer drives will continue to have the pre-amplifier mounted on the flex cable, as close to the head as possible. In the future, the pre-amplifier may be mounted directly on the head. For performance and size reasons, flip-chip is the method of choice for interconnecting the die to the flex circuit. Typical flex assemblies are shown in Figure 1, and the pre-amplifier IC is mounted at the tip of each flex assembly. Assembly Considerations The flex circuit assembly poses several challenges. First, high accuracy requirements are driven by the use of flip-chip. Although a high percentage of today's flip-chip volumes have pitches equal to or greater than 200 microns, the trend is clearly toward tighter pitch arrays in the range of 150-180 microns. With such tight pitch arrays, high accuracy linear motor based placement machines are used for assembly. The following simple analysis provides an accuracy guideline:
Although the accuracy of the base linear motor system is critical, other factors also play a major role in the overall performance of the die bonder, such as camera systems, vision algorithms and illumination schemes, which must be developed to handle precise placement needs. Illumination, for example, is challenging because of the low level of contrast between the copper circuit and the supporting polyimide flex material. Traditional illumination approaches no longer provide the functionality that permit reliable placement. Effective lighting of the circuit is critical, since even the best die placement machine is paralyzed if the circuit cannot be reliably imaged. Accurate Placement Fluxing of the die and high-accuracy placement of bumped devices require functionality that is typically found on a flip-chip bonder, such as dip fluxing, and the use of waffle pack or wafer die feeding techniques. Suitable nozzles and tooling are essential to ensure accurate processing of the device once the imaging of the die has taken place. Finally, placement of passive 0402 components, as well as the placement of the connector, is also required. The low number of passives (5-10), and the singulated nature of the substrate are not well suited to the use of traditional passive placement equipment. The solution to all these challenges must, of course, take into account the cost sensitivity of this market segment. The placement system must, therefore, provide high machine throughput along with the flexibility to handle future generations. Handling Strategies Automation strategies addressing these challenges must focus on handling all types of components and the special requirements associated with flip-chip, in addition to the basic accuracy requirements. And, optimum cost per placement must remain a critical objective. The basic platform machine used for these types of applications is a small footprint, linear motor system with a specified accuracy of 24 microns at 6 sigma. The use of 1 micron linear encoders and dual Y-axis motors permit this level of accuracy.
Imaging Issues To address the imaging issues associated with flex assemblies, traditional lighting systems must be reworked because they are not optimum for these assemblies. An example of this is the recent introduction of a novel illumination module.3 With this module, lighting parameters are established to provide optimum contrast between the copper circuit and the underlying polyimide material. Improvements in image contrast are made possible by taking advantage of the light transmission properties of these materials. When using a conventional monochromatic (~660 nm) lighting system, the metal traces of the pad site are virtually indistinguishable from the surrounding polyimide. This is because polyimide is very transmissive to the red light of the illumination module. In the background areas surrounding the metal traces and fiducials, the red light is transmitted through the polyimide and reflects from the metal backing of the circuit, resulting in a bright background. The copper features on the substrate also reflect the red light efficiently. The result is a bright feature on a bright background-a low contrast image. The quality of the image shown in Figure 2 was achieved with the new illumination module equipped with blue LEDs (~470 nm). Since polyimide strongly absorbs the blue portion of the spectrum, the previously bright background is now dark. The copper features on the substrate reflect the blue light efficiently. The result is a significant improvement in image quality. Machine Configuration A unique machine configuration has been introduced to address the challenges of high-throughput placement of flip-chips requiring high accuracy, and passive components using a single placement cell. In this mixed head configuration, a high accuracy placement head with four spindles is used for fluxing, die inspection, and placement of the flip chip devices and the connector. In addition, a high-speed head with seven spindles is used for the 5-10 passive components that are assembled on a typical flex cable. The high accuracy head can handle the most challenging flip-chip applications, while the high speed head provides high-throughput chip placement. The mixed head configuration is illustrated in Figure 3. Figures 4 and 5 show the high accuracy placement head and the high speed placement head, respectively. (Typical throughputs obtained with such a configuration compared with a single head machine are shown in Figure 6.)
Conclusion The emergence of flip-chip as the method of choice for flex assembly has introduced many new challenges to the assembler. These challenges have, in turn, driven equipment manufacturers to develop novel, high volume solutions to meet new requirements. The development of high accuracy equipment, special imaging techniques, and high speed placement solutions have permitted manufacturers to automate the assembly of these circuits in a cost-effective manner. References 1. J. W. Toigo, "Avoiding a data crunch," Scientific American, May 2000, p. 59-74. 2. IBM web storage web site [storage.ibm.com/technology] 3. 1999 Rigid Disk Drive Report, Disk/Trend Inc., May 2000 4. J. W. Herman, J. Radice, et al., "Flexible Lighting Modules," Advanced Packaging, May 2000, p. 27-32.
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